Rotary positive displacement compressor and refrigeration plant

ABSTRACT

The rotary positive displacement compressor has an intermediate port (42) for fluid from an intermediate pressure channel (30) and has a bleed port (44) for recirculation of partly compressed fluid through a return channel (32). It is suggested to provide the compressor with valve means (36), selectively adjustable between two end positions. In a first end position it opens up a direct communication between these channels (30, 32) and opens the bleed port (44), whereby fluid flows directly from the intermediate pressure channel (30) to the return channel (32) simultaneously as fluid within the working space of the compressor flows to the return channel (32) through the intermediate port (42) as well as through the bleed port (44). In a second end position the bleed port (44) is closed and said direct communication is blocked. The disclosure also relates to a refrigeration plant comprising such a compressor.

BACKGROUND OF THE INVENTION

The present invention relates to a rotary positive displacementcompressor comprising at least one rotor forming compression chambers ina working space, the compressor having an inlet port communicating witha low pressure channel, an outlet port communicating with a highpressure channel, intermediate port means communicating with anintermediate pressure channel and bleed port means selectivelyconnectable to said low pressure channel through a return channel, saidintermediate port means and said bleed port means being located suchthat they face a compression chamber within said working space in acompression chamber, which chamber is sealed from communication withsaid inlet port as well as said outlet port by said at least one rotor.

The invention further relates to a plant of refrigeration typecomprising such a compressor and having a condenser communicating withsaid high pressure channel, an evaporator communicating with said lowpressure channel, a vessel for an intermediate pressure communicatingwith said intermediate pressure channel, a channel connecting saidcondenser to said vessel, said channel having first pressure reducingmeans for decreasing the high pressure in said condenser to theintermediate pressure in said vessel and a channel connecting saidvessel to said evaporator, said channel having second pressure reducingmeans decreasing the intermediate pressure in said vessel to the lowpressure in said evaporator.

A compressor and a plant of such types are earlier known from U.S. Pat.No. 3,913,346. The intermediate pressure zone in such plants is used forinternal cooling purposes within the plant at a temperature level abovethat of the evaporator. The main cooling purpose is to precool theliquified refrigerant before the supply thereof to the evaporator whichresults in a more effective use of the evaporator area so that thedimensions thereof can be minimized for a certain capacitysimultaneously as the swept volume of the compressor and thus itsdimensions can be reduced correspondingly. Furthermore the powerrequired for recompression of the gaseous refrigerant supplied at theintermediate pressure will be less than that if all the refrigerant weresupplied at the evaporator pressure.

In order to vary the volumetric capacity the compressor in U.S. Pat. No.3,913,346 is provided with a selectively adjustable valve controlling ableed port in the wall of the working space so that a certain amount ofthe working fluid supplied to the compressor may be returned to theinlet channel of the compressor. This bleed port is disposed within thesame phase of the compression cycle as the intermediate port means. Whenthe bleed port is opened the pressure level inside the compressorworking space decreases to such an extent that the back pressure withinthe area of the intermediate port means will be practically the same asthat in the low pressure channel. The bleed port must in order to avoidthrottling losses be provided with a large area corresponding to what isrequired not only for the recirculation of the surplus fluid suppliedthrough the inlet port but also for draining the fluid supplied throughthe intermediate port means. The size of the valve member will thus betoo large for location in the end wall with regard to its area comparedwith the limited space available outside the rotor bearings. For thisreason the valve has to be located in the barrel wall of the workingspace. Such a valve will consequently be complicated in shape andexpensive to manufacture as it not only has to sealingly cooperate withits seat in the housing but also has to sealingly cooperate with theconfronting rotor or rotors in order to avoid internal leakage withinthe compressor, especially when running under maximum capacityconditions.

In the PCT-application with International Publication Number W086/06798a compressor is disclosed, where the discussed problems related to acompressor and a refrigeration plant of the type in question areovercome by providing a connection controlled by a selectivelyadjustable over-flow valve between the intermediate pressure channel andthe low pressure channel. In this way the need for a separate bleed portis eliminated as the intermediate port means will act as such a portduring low volumetric capacity conditions when only the surplus suppliedworking fluid has to be drained from the working space.

The main object of the present invention is to reach an alternativesolution to overcome these problems so as to achieve a more effectivecapacity control of the compressor per se as well as of a completerefrigeration plant by means of simpler and less expensive valvearrangement than those used in the prior art.

According to one aspect of the invention this object is attained byproviding a compressor of the introductionally specified kind with valvemeans, selectively adjustable between two end positions for formation ofdifferent flow paths within the compressor, in the first end positionsaid valve means opens up a direct communication between saidintermediate pressure channel and said return channel and opens saidbleed port means, whereby fluid flows directly from the intermediatepressure channel to the return channel simultaneously as fluid withinthe working space flows to the return channel through the intermediateport means as well as through the bleed port means, whereas in thesecond end position said valve means blocks said direct communicationbetween said intermediate pressure channel and said return channel andcloses said bleed port means.

According to another aspect of the invention this object is attained byproviding a refrigeration plant of the introductionally specified kindwith valve means as specified above.

The main advantage with a compressor and a refrigeration plant accordingto the invention is the possibilty to optimize the areas of the bleedport means and the intermediate port means, thereby allowing greaterfreedom for their location and admitting less complicated valveconstructions for the bleed port means. The area of the intermediateport means is determined only by what is required for the passage of theintermediate pressure fluid from the intermediate pressure channel tothe compressor. At reduced capacity condition when the valve means is inthe first end position a part of the partly compressed fluid which is tobe recirculated to the inlet flows through the intermediate port meansto the return channel. The bleed port means thus can be dimensioned totake care of only the remaining part of the fluid to be recirculated.

Further objects of the invention and how those are met will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the scope of the invention will become apparent tothose skilled in the art from this detailed description.

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawings

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagramatically illustrates an embodiment of a refrigerationplant according to the invention,

FIG. 2 is a schematic section through a compressor according to theinvention,

FIG. 3 is a detailed section through a part of a compressor according tothe invention showing the valve means in the second end position,

FIG. 4 is a section similar to FIG. 3, but showing the valve means inthe first end position,

FIG. 5 is a section taken along line V--V in FIG. 3, and

FIG. 6 is a section similar to FIG. 5, but showing another embodiment.

DETAILED DESCRIPTION

A refrigeration plant as shown in FIG. 1 comprises a compressor 10communicating with a condenser 12 through a high pressure channel 18connected to the outlet port 40 of the compressor and with an evaporator16 through a low pressure channel 24 connected to the inlet port 38 ofthe compressor. The condenser 12 and the evaporator 16 areinterconnected by channels 20, 22 in which two sets of pressurereduction means 26, 28 are disposed, each shaped as a throttling valve.An intermediate pressure vessel 14 in the shape of a flash chamber isdisposed between the two throttling valves 26, 28. The flash gas side ofthe intermediate pressure vessel 14 communicates through an intermediatepressure channel 30 with intermediate port means 42 in the compressor10.

The compressor 10 is provided with a return channel 32 ending in a bleedport 44 in the compressor and communicating with the low pressurechannel 24. A branch channel 34 connects the intermediate pressurechannel 30 and the return channel 32. A valve 36 is provided in thereturn channel 32, where the branch channel 34 ends in the returnchannel. The valve 36 has two end positions. In the first endpositionthe bleed port 44 is in communication with the low pressure channel 24through the return channel 32, and in this position the branch channel34 communicates with the return channel 32. In the second end positionof the valve, communication through the return channel 32 is broken andthe branch channel 34 does not communicate with the return channel 32.

The compressor 10, schematically shown in FIG. 2, is of the intermeshingscrew type having a male rotor 54 and a female rotor 56, the male rotor54 being driven by a motor 72. Each rotor is provided with helical lobesand intermediate grooves, through which the rotors 54, 56 intermesh,forming chevron-shaped compression chambers. The rotors are working in aworking space 58 limited by a low pressure end section 60, in which theinlet port 38 is located, a high pressure end section 62, in which theoutlet port 40 is located and a barrel section 64 extendingtherebetween.

The intermediate port means 42 is located in the barrel section 64 andthe bleed port means 44 in the high pressure end section 62. These portmeans 42, 44 face the working space 58 in the same stage of thecompression cycle, when the compression chamber by the rotors 54, 56 isclosed off from communication with the inlet port 38 as well as with theoutlet port 40.

FIGS. 3 and 4 show the bleed port means 44 and the intermediate portmeans 42 more in detail and how they cooperate with the selectivelyadjustable valve means 36 in the two positions thereof. The valve means36 comprises a cylindrical valve member 46 displaceable in a bore 48 inthe high pressure end section 62. One end of said bore 48 partly facesthe working space 58, thereby forming the bleed port means 44, andpartly is covered by the end surface 66 of the barrel section 64. Theintermediate pressure channel 30, ending in the intermediate port means42 is radially disposed in the barrel section 64. An axially directedbranch channel 34 leads from the intermediate pressure channel 30 to thepart of the barrel section end surface 66 covering a part of the bore 48and faces the bore 48 through a first opening 68. The return channel 32is radially disposed in the high pressure end section 62 and ends in thecircumference of the bore 48 through a second opening 70. At the rearside of valve member 46 a pipe 50 for actuation fluid ends in the bore48. This pipe 50 can be connected to either a high pressure source or alow pressure source. By a spring 52 the valve member 46 is biasedtowards its first end position.

A refrigeration plant according to the invention operates in thefollowing way. Compressed gaseous working fluid is delivered from thecompressor 10 to the condenser 12 where it is liquified by externalcooling means. From the condenser 12 the liquified working fluid passesthrough the first throttling valve 26, whereby the pressure is reduced,to the intermediate pressure vessel 14 where the working fluid is partlyevaporated as flash gas and the remaining liquified working fluid iscooled down to the evaporating temperature corresponding to the pressurein the intermediate pressure vessel 14. This cooled liquified workingfluid passes through the second throttling valve 28 whereby the pressureis further reduced, to the evaporator 16 where the working fluid isevaporated by external heating means. The low pressure gaseous workingfluid is then returned from the evaporator 16 to the compressor 10 inlet38, recompressed and delivered to the condenser 12. The flash gasproduced in the intermediate pressure vessel 14 is passed on to theintermediate pressure channel 30 communicating with the intermediateport means 42 in the wall of the working space 58 of the compressor 10.

At full capacity conditions of the plant the adjustable valve means 36is in its second end position, in which there is no recirculation ofworking fluid from the bleed port means 44 to the low pressure channel24, and in which the intermediate pressure fluid in the intermediatepressure channel cannot pass from the branch channel 34 to the returnchannel 32. The compressor 10 is filled to its maximum capacity by lowpressure working fluid from the evaporator 16 through the inlet port 38simultaneously as the intermediate pressure gas is supplied through theintermediate port means 42 to a compression chamber where the pressurehas already been increased from the inlet port conditions. In this waythe power for recompression of the gas supplied through the intermediateport means 42 is reduced as the compression thereof starts at a higherpressure level than the inlet pressure of the compressor. Simultaneouslythe full capacity of the compressor can be used for the gas from theevaporator which means that for a certain capacity of the plant thedimensions of the compressor can be reduced.

In order to achieve part load condition the valve means 36 is actuatedto its first end position, forming communication between the bleed portmeans 44 and the low pressure channel 24 through the return channel 32and forming communication between the branch channel 34 and the returnchannel 32. The fluid coming from the intermediate pressure vessel 14thereby flows from the intermediate pressure channel 30 through thebranch channel 34 to the return channel 32 and further to the lowpressure channel 24. Simultaneously partly compressed fluid flows fromthe working space 58 to the low pressure channel via two different flowpaths. One of them goes through the bleed port 44 and the return channel32. The other one goes through the intermediate port means 42, thebranch channel 34 and the return channel 32. The working fluid returnedto the low pressure channel 24 replaces some of the gas otherwise suckedin from the evaporator 16 and thus reduces the capacity of thecompressor so that the capacity of the plant is reduced. Since the bleedport means 44 has to take care of only a part of the working fluid to berecirculated, as a part thereof can pass through the intermediate portmeans 42, the opening area of the bleed port means 44 can beconsiderably reduced in comparence with known technique.

The function of the valve means 36 in a preferred embodiment of theinvention can be understood from the detailed FIGS. 3 and 4. FIG. 3, inwhich the valve means 36 is in the second end position, illustrates theconditions when the compressor is running at full capacity. The flow ofthe intermediate pressure fluid through the intermediate pressurechannel 30 and the intermediate port means 42 into the working space 58of the compressor is indicated by arrows. It can be seen in the figurehow in this position the front end surface of the valve member 46 coversthe bleed port 44 and the first opening 68, where the branch channel 34ends in the bore 48, and how the cylindrical surface of the valve member46 covers the second opening 70, where the return channel reaches thebore 48. Thus no fluid is recirculated through the return channel 32,neither from the bleed port means 44, nor from the intermediate pressurechannel 30. The valve member 46 is kept in the second end position byhaving: the pipe 50 connected to a high pressure source. This highpressure acts on the rear side of the valve member 46 against the actionof the spring 52 and against the pressure acting on the front sidethereof.

When the compressor is to be operated under part-load condition, thevalve member 46 is actuated to the first end position, shown in FIG. 4,by connecting pipe 50 to a low pressure source. In this position theworking space 58, the branch channel 34 and the return channel 32 allcommunicate with the bore 48 through the bleed port means 44, the firstopening 68 and the second opening 70, respectively. As indicated by thearrows, fluid from the intermediate pressure channel 30 passes throughthe branch channel 34 to the bore 48, simultaneously as fluid in theworking space 58 flows to the bore 48 partly through the bleed portmeans 44, partly through the intermediate port means 42 and the branchchannel 34. From the bore 48 the fluid passes through the second opening70 to the return channel 32 and further to the low pressure channel 24.

To avoid throttling losses the area of the first opening 68 should belarger than the area of the intermediate port means 42, and the area ofthe second opening 70 should be larger than the area of the firstopening 68. By the same reason the area of the second opening 70 shouldexceed or equal the sum of the areas of the bleed port means 44 andfirst opening 68.

FIG. 5 shows the locations of the openings facing the bore 48 as seen ina section taken along line V-V in FIG. 3.

FIG. 6 illustrates in a corresponding section an alternative embodimentof how these openings and the channels connected thereto can bearranged. In this embodiment also the return channel 32' is disposedaxially in the barrel section 64 and ends axially in the bore 48 throughthe second opening 70'.

I claim:
 1. A rotary positive displacement compressor (10) comprising:atleast one rotor (54, 56) forming compression chambers in a working space(58); an inlet port (38) communicating with a low pressure channel (24);an outlet port (40) communicating with a high pressure channel (18);intermediate port means (42) communicating with an intermediate pressurechannel (30); bleed port means (44) selectively connectable to said lowpressure channel (24) through a return channel (32); said intermediateport means (42) and said bleed port means (44) being located such thatthey face a compression chamber within said working space (58), whichcompression chamber is sealed from communication with said inlet port(38) as well as from said outlet port (40) by said at least one rotor(54, 56); and valve means (36), selectively adjustable between two endpositions for formation of different flow paths, wherein in said firstend position, said valve means (36) opens up a direct by-passcommunication between said intermediate pressure channel (30) and saidreturn channel (32) and opens said bleed port means (44), forming afirst flow path as a by-pass flow path between said intermediatepressure channel (30) and said return channel (32), a second flow pathbetween said working space (58) and said first flow path through saidintermediate port means (42) and a third flow path between said workingspace (58) and said return channel through said bleed port means (44),and wherein in said second end position, said valve means (36) blockssaid direct communication between said intermediate pressure channel(30) and said return channel (32) and closes said bleed port means (44),forming a flow path between said intermediate pressure channel (30) andsaid working space (58) through said intermediate port means (42). 2.Compressor according to claim 1, comprising two of said rotors (54, 56),each rotor (54, 56) being provided with helical lobes and intermediategrooves, through which said rotors (54, 56) intermesh, formingchevron-shaped compression chambers; andsaid working space (58) havingthe form of two intersecting circular cylinders and being limited by ahigh pressure end section (62), a low pressure end section (60) and abarrel section (64) extending therebetween.
 3. Compressor according toclaim 2, in which said intermediate port means (42) is disposed in saidbarrel section (64) and said bleed port means is disposed in said highpressure end section (62).
 4. Compressor according to claim 3, in whichsaid selectively adjustable valve means (36) is disposed in said highpressure end section (62) and comprising a cylindrical valve member (46)displaceable in a bore (48), one end of said bore (48) partly facingsaid working space (58) and partly being covered by the adjacent endsurface (66) of said barrel section (64), the part facing the workingspace constituting said bleed port means (44), said intermediatepressure channel (30) communicating with said bore (48), through a firstopening (68), said return channel (32, 32') ending in said bore (48)through a second opening (70, 70'), which valve member (46) in the firstend position of the valve means uncovers said bleed port means (44) andsaid first (68) and second (70, 70') openings allowing working fluid toflow from said bleed port means (44) and said first opening (68) to saidsecond opening (70, 70') and which valve member (46) in the second endposition of the valve means covers said bleed port means (44) and saidfirst (68) and second (70, 70') openings preventing any communicationtherebetween.
 5. Compressor according to claim 4, in which said first(68) and second (70') openings are disposed in said end surface (66) ofthe barrel section (64) which partly covers said one end of said bore(48).
 6. Compressor according to claim 5, in which said first opening(68) has a larger area than said intermediate port means (42), and saidsecond opening (70, 70') has a larger area than said first opening (68).7. Compressor according to claim 6, in which the area of said secondopening (70, 70') is at least as large as the sum of the areas of saidfirst opening (68) and said bleed port means (44).
 8. Compressoraccording to claim 5, in which said valve member (46) is actuated byfluid pressure.
 9. Compressor according to claim 4, in which said firstopening (68) is disposed in said end surface (66) of the barrel section(64), which covers said one end of said bore (48), and said secondopening (70) is radially disposed in said bore (48).
 10. Compressoraccording to claim 9, in which said first opening (68) has a larger areathan said intermediate port means (42), and said second opening (70,70') has a larger area than said first opening (68).
 11. Compressoraccording to claim 10, in which the area of said second opening (70,70') is at least as large as the sum of the areas of said first opening(68) and said bleed port means (44).
 12. Compressor according to claim9, in which said valve member (46) is actuated by fluid pressure. 13.Compressor according to claim 4, in which said first opening (68) has alarger area than said intermediate port means (42), and said secondopening (70, 70') has a larger area than said first opening (68). 14.Compressor according to claim 13, in which the area of said secondopening (70, 70') is at least as large as the sum of the areas of saidfirst opening (68) and said bleed port means (44).
 15. Compressoraccording to claim 4, in which said valve member (46) is actuated byfluid pressure.
 16. A refrigeration plant comprising:a rotary positivedisplacement compressor (10); a condenser (12) communicating with anoutlet port (40) of said compressor through a high pressure channel(18); an evaporator (16) communicating with an inlet port (38) of saidcompressor through a low pressure channel (24); a vessel (14) for anintermediate pressure communicating with intermediate port means (42) ofsaid compressor (10) through an intermediate pressure channel (30); afirst channel (20) connecting said condenser (12) to said vessel (14),said channel (20) having first pressure reduction means (26) fordecreasing a high pressure in said condenser (12) to the intermediatepressure in said vessel (14); a second channel (22) connecting saidvessel (14) to said evaporator (16), said second channel (22) havingsecond pressure reduction means (28) for decreasing an intermediatepressure in said vessel (14) to the low pressure in said evaporator;said compressor (10) having at least one rotor (54, 56) formingcompression chambers in a working space (58) and having bleed port means(44) selectively connectable to said low pressure channel (24) through areturn channel (32); said intermediate port means (42) and said bleedport means (44) being located such that they face a compression chamberwithin said working space (58), which compression chamber is sealed fromcommunication with said inlet port (38) as well as from said outlet port(40) by said at least one rotor (54, 56); and valve means (36),selectively adjustable between two end positions, for formation ofdifferent flow paths, wherein in said first end position, said valvemeans (36) opens up a direct communication between said intermediatepressure channel (30) and said return channel (32) and opens said bleedport means (44), forming a first flow path as a by-pass flow pathbetween said intermediate pressure channel (30) and said return channel(32), a second flow path between said working space (58) and said firstflow path through said intermediate port means (42) and a third flowpath between said working space (58) and said return channel throughsaid bleed port means (44), and wherein in said second end position,said valve means (36) blocks said direct communication between saidintermediate pressure channel (30) and said return channel (32) andcloses said bleed port means (44), forming a flow path between saidintermediate pressure channel (30) and said working space (58) throughsaid intermediate port means (42).